An ongoing trend in the manufacture of electronic equipment is a reduction in the size of electronic components, and especially that of Surface Mount (SMT) components used in low power high speed digital circuitry (e.g., Dual In-Line Memory Modules, or DIMMs). This is a welcome trend that cooperates with increased functionality of associated Integrated Circuits (ICs) to produce Printed Circuit Boards (PCBs) of high component densities and extensive capabilities.
On the other hand, these reductions in component size are often accompanied by a corresponding increase in lead pitch (number of leads per unit of distance), which is to say, by a corresponding reduction in the center-to-center spacing between adjacent peripheral leads on a package. This complicates the task of probing the signals on those leads; old style probes may have probe tips that are simply too bulky to deploy without shorting adjacent leads, not to mention the difficulty of maintaining contact by hand. Often, the acquisition of a single signal is not sufficient. Many high speed signals are differential, and require probing two leads, in addition to a ground. For the highest pitches the use of ‘grabbers’ is not feasible, and the industry has resorted to simply soldering the ‘flying leads’ (short flexible wires) of probes onto the leads to be probed. In the field of logic analysis, a Logic Analyzer often needs to acquire a great many signals at one time, which compounds the problem many times over. Clearly, what is required in these situations is an improved probing technique where the probe tips are attached or otherwise retained in contact with their associated leads.
As we consider what might be done to accomplish such improved probing, we are also reminded that the practice of placing a PCB onto extender boards to expose its components to a relatively unconfined space from within which the human hand can approach them with a probe is, for high speed circuitry, generally not practical: the circuitry on the PCB often simply will not perform properly when connected via such an artificial environment as an extension board. Evidently, a suitable probing technique for such circuits is also one that can remain deployed when the PCB of interest is plugged into its native electrical environment, even though it might have to be temporarily removed therefrom to attach or remove the probe. The implication is that the probe is flexible and has a low profile, as PCBs are often placed in parallel arrays that are rather close together.
Finally, further reflection or experimentation reveals that such small component leads offer very little surface area for solder to join an individual probe tip to a lead, meaning that the solder joint is fairly fragile, as soldered joints go.
Another consideration becomes important when the high speed nature of the signals to be probed is considered. For fast signals it is imperative that there be a minimum of probe tip length that separates the probed component lead and the working components within the probe proper, so as to minimize the introduction of stray inductance and stray capacitance. Often, the initial working component within a probe is an isolation or damping resistance, so that achieving a minimal probe tip length amounts to getting the isolation resistance (or other initial working component) relatively close to the location being probed. This is important for two related reasons: first, it aids in observing with fidelity the (as probed) signal of interest as it occurs; and, second, it minimizes loading so that the signal as probed is essentially the same as it was before it was probed.
Various prior art techniques have been developed to probe a PCB's components, and some of them are more convenient or more reliable than others. There have been ‘IC clips’ that compress contacts onto leads on opposing sides of a part, ‘clothes pin’ style as it were, while offering an easy to probe stud or hard wired connection at the other end of the contacts. The geometry of these devices makes them bulky and not suitable for use with high speed high pitch parts. They also greatly extend the effective length of the probe tip, rendering them unsuitable for most high speed use. The same may be said of the ‘clip-on grabber’ that accepts the central core of a 'scope probe and that is described in U.S. Pat. No. 4,949,032. There have also been probing schemes where the traces on a PCB (say, for a bus of interest) have been extended to reach a conveniently located connector provided for the purpose of receiving a mating connector carried by a cable originating with the test equipment. This can often be made to work with proper attention to adequate drive and proper termination of transmission lines. But it is an expensive solution that is not economical for the various other signals that are on the PCB and that might need probing. There have been ‘wedge’ probes as described in U.S. Pat. No. 5,463,324 (a version for a quad flat pack IC) and U.S. Pat. No. 5,923,177 (a ‘browser’ version for a small number of signals). They are relatively expensive, fragile, and are definitely not low profile. The latter means that it is often not possible to use them when the PCB of interest is operated in a native environment of several closely spaced and parallel PCBs plugged into a chassis or motherboard. Nor can wedge probes be used in cases where there is no exposed parallel lead surfaces for the part, such as for BGAs (Ball Grid Arrays whose interconnecting leads are beneath the part) or certain surface mount multiple resistor packs (R-Packs) where the substrate material extends into the space between adjacent leads or where the leads themselves are simply very thin plated traces. There have been various high frequency ‘browser’ oscilloscope probes intended to be soldered to single ended or differential signals, but they are not suitable for the acquisition of several signals at once.
In summary, the techniques used to probe components having external peripheral leads soldered into through-holes (vias) are generally for moderate to widely spaced leads, and are not suitable for surface mounted components. Techniques developed for probing multiple signals at once for through-hole leads are not extensible for use with the denser lead spacing found on many SMT components. Thus, there is a need for an economical and reliable probing technique that is suitable for several closely spaced signals at once, that is of a flexible low profile and that does not introduce any unnecessary length in the path between the location of probing the signal and the first working component in the probe. We might say that there is a need for high fidelity in-situ probing of signals for SMT parts located on PCBs that are operated in their native environment. It would be desirable if such a new technique were also one that continued to lend itself for use with older style peripheral leads mounted in through holes. What to do?